Nucleo-cytoplasmic control of perithecial formation in Aspergillus nidulans

Abstract

A spontaneous mutant colony alba, being aperithecial and having colourless mycelium, arises from spores of four wild-type strains of Aspergillus nidulans. Its analysis gives a clear-cut demonstration of non-Mendelian inheritance. The frequency of alba under standard conditions is different in each strain, the difference being under the control of a nuclear gene f, three alleles of which have been identified and its position mapped on the y chromosome. Two possible models explaining the experimental results are presented, the first assuming the presence of a replicating cytoplasmic particle interacting with the f gene. Where the relative amount of particles is below a certain threshold value (this being determined by f), alba colonies result under standard conditions. These revert to normal on appropriate treatment. Stable alba mutants arise when the particle is lost to the cell altogether. The second postulates a reaction cycle in which the heterocatalytic action of each member of the cycle insures the reproduction of the catalysts themselves in a pseudo-autocatalytic system. Loss of one of the constitutent units would cause the breakdown of the system and this would be reflected in the production of alba colonies. The f gene is seen as having a regulatory function in the system, different f alleles giving different reaction rates. Restoration of cytoplasmic deficiency can be made only by a ‘transfusion’ of normal cytoplasm following hyphal fusion. Two genes affecting other aspects of perithecial formation are described, one determining a condition of self-sterility and the other controlling perithecial density.